The present invention relates to a camera provided with an automatic focusing system wherein an objective lens assembly of the camera is automatically shifted in an axial direction thereof to an optimum position for focusing a target object to be photographed on a focal plane of the camera, in accordance with a control signal produced from a distance detection device and being indicative of a distance between a target object to be photographed and the camera and, more particularly, to means provided in the automatic focusing system for disabling the system when no control signal is produced from the distance detection device.
Conventionally, there have been proposed various types of cameras provided with the automatic focusing system coupled with a camera operation disabling means, one of which is disclosed in U.S. Pat. No. 3,442,193 to A. B. Pagel. The automatic focusing system disclosed therein includes a movable objective lens which moves between a hyperfocal or infinity position and near position relative to the focal plane and a rangefinding means having a member positionable automatically as a function of the distance from the camera to a target object to be photographed. Both of the movable objective lens and rangefinding means are coupled to a suitable motor means, and are actuated in relation to a movement of a shutter release mechanism, particularly by the application of a finger pressure over a shutter button.
When the rangefinding means suceeds in finding the distance between the camera and the target object, as the result of actuation of the rangefinding means together with the objective lens, the objective lens is locked in a properly focused position, and, in turn, the shutter is released.
On the other hand, when the rangefinding means fails to find the camera to object distance due to the fact that the target object is too dark to be detected or the portion to be focused is a plain wall having no distinctive contrast, or by some other reasons, the objective lens will be shifted upon its final position, that is, near position, and, in turn, the motor actuating the movable objective lens is disconnected from a source of electric power. Thereafter, upon return of the shutter release mechanism to its original position incident to, for example, the removal of the finger pressure from the shutter button, the motor runs in a reverse direction to return the objective lens and rangefinding means to their original positions.
In other words, the failure of the rangefinding means to find the camera to object distance is not noticed until the objective lens is moved to the near position. This movement of the objective lens to the near position as a result of the failure of the rangefinding means to find the distance is not only useless but also allows the motor to consume electric power unnecessarily. Furthermore, it is necessary to provide means for returning the objective lens to the initial position, that is, hyperfocal position, for the purpose of allowing successive operation of the automatic focusing system.
Another type of automatic focusing system is disclosed in U.S. Pat. No. 3,945,023 to Stauffer wherein the rangefinding means disclosed therein the first and second lenses aligned in side-by-side relation to each other and a plurality of photoresponsive elements disposed behind each of the lenses for comparing a signal, obtained from the photoresponsive elements provided behind the first lens, with a signal obtained from the photoresponsive elements provided behind the second lens. When this rangefinding means fails to find the distance between the camera and the target object, the movable objective lens employed in the camera is prevented from being shifted.
However, in general, since the above described type of rangefinding means includes a capacitance component in a circuit and/or photoresponsive element, the time interval between the start of supply of an electric power and the generation of the control signal from the rangefinding means is considerably long. Therefore, it is necessary to employ a delay means for delaying the start of movement of the objective lens until the capacitance component is charged upto a desired voltage.